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The epididymis (; plural: epididymides or ) is a tube that connects a testicle to a vas deferens in the male reproductive system. It is a single, narrow, tightly-coiled tube in adult humans, in length. It serves as an interconnection between the multiple efferent ducts at the rear of a testicle (proximally), and the vas deferens (distally).


Anatomy

The epididymis is situated posterior and somewhat lateral to the testis. The epididymis is invested completely by the tunica vaginalis (which is continuous with the tunica vaginalis covering the testis). The epididymis can be divided into three main regions: * The head ( la, caput). The head of the epididymis receives spermatozoa via the efferent ducts of the mediastinium of the testis at the superior pole of the testis. The head is characterized histologically by a thick
epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
with long stereocilia (described below) and a little
smooth muscle Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...
. It is involved in absorbing fluid to make the
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
more concentrated. The concentration of the sperm here is dilute. * The body ( la, corpus, links=no). This has an intermediate epithelium and smooth muscle thickness. * The tail ( la, cauda, links=no). This has the thinnest epithelium of the three regions and the greatest quantity of smooth muscle. The tail is distally continuous with (the convoluted portion of) the ductus deferens (s. vas deferens).


Histology

The epididymis is covered by a two layered
pseudostratified epithelium A pseudostratified epithelium is a type of epithelium that, though comprising only a single layer of cells, has its cell nuclei positioned in a manner suggestive of stratified epithelia. As it rarely occurs as squamous or cuboidal epithelia ...
. The epithelium is separated by a
basement membrane The basement membrane is a thin, pliable sheet-like type of extracellular matrix that provides cell and tissue support and acts as a platform for complex signalling. The basement membrane sits between Epithelium, epithelial tissues including mesot ...
from the
connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
wall which has smooth muscle cells. The major cell types in the epithelium are: * Principal cells: columnar cells that, with the basal cells, form the majority of the epithelium. In the caput (head) region these cells have long stereocilia that are tuft-like extensions that project into the lumen. The stereocilia are much shorter in the cauda (tail) segment. They also secrete
carnitine Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria. In support of energy metabolism, carnitine transports long-chain fatty acids into mitochondria to be oxidized for energy production, an ...
,
sialic acid Sialic acids are a class of alpha-keto acid sugars with a nine-carbon backbone. The term "sialic acid" (from the Greek for saliva, - ''síalon'') was first introduced by Swedish biochemist Gunnar Blix in 1952. The most common member of this gr ...
, glycoproteins, and glycerylphosphorylcholine into the lumen. * Basal cells: shorter, pyramid-shaped cells which contact the basal lamina but taper off before their apical surfaces reach the lumen. These are thought to be undifferentiated precursors of principal cells. * Apical cells: predominantly found in the head region * Clear cells: predominant in the tail region * Intraepithelial lymphocytes: distributed throughout the tissue. * Intraepithelial macrophages


Stereocilia

The stereocilia of the epididymis are long cytoplasmic projections that have an actin filament backbone. These filaments have been visualized at high resolution using fluorescent phalloidin that binds to actin filaments. The stereocilia in the epididymis are non-motile. These membrane extensions increase the surface area of the cell, allowing for greater absorption and secretion. It has been shown that epithelial sodium channel
ENaC The epithelial sodium channel (ENaC), (also known as amiloride-sensitive sodium channel) is a membrane-bound ion channel that is selectively permeable to sodium ions (). It is assembled as a heterotrimer composed of three homologous subunits α ...
that allows the flow of Na+ ions into the cell is localized on stereocilia. Because sperm are initially non-motile as they leave the seminiferous tubules, large volumes of fluid are secreted to propel them to the epididymis. The core function of the stereocilia is to resorb 90% of this fluid as the spermatozoa start to become motile. This absorption creates a fluid current that moves the immobile sperm from the seminiferous tubules to the epididymis. Spermatozoa do not reach full motility until they reach the vagina, where the alkaline pH is neutralized by acidic vaginal fluids.


Development

In the
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
, the epididymis develops from tissue that once formed the mesonephros, a primitive kidney found in many aquatic vertebrates. Persistence of the cranial end of the mesonephric duct will leave behind a remnant called the ''
appendix of the epididymis The appendix of the epididymis (or pedunculated hydatid) is a small stalked appendage (sometimes duplicated) on the head of the epididymis. It is usually regarded as a detached efferent duct. This structure is derived from the Wolffian duct (Meson ...
''. In addition, some mesonephric tubules can persist as the '' paradidymis'', a small body caudal to the
efferent ductules The efferent ducts (or efferent ductules or ductuli efferentes or ductus efferentes or vasa efferentia) connect the rete testis with the initial section of the epididymis.Hess 2018 There are two basic designs for efferent ductule structure: * ...
. A Gartner's duct is a
homologous Homology may refer to: Sciences Biology *Homology (biology), any characteristic of biological organisms that is derived from a common ancestor *Sequence homology, biological homology between DNA, RNA, or protein sequences * Homologous chrom ...
remnant in the female.


Function


Role in storage of sperm and ejaculant

Spermatozoa formed in the testis enter the caput epididymis, progress to the corpus, and finally reach the cauda region, where they are stored. Sperm entering the caput epididymis are incomplete—they lack the ability to swim forward (motility) and to
fertilize Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...
an egg. Epididymal transit takes 2 to 6 days in humans and 10–13 in rodents. During their transit in the epididymis, sperm undergo maturation processes necessary for them to acquire motility and fertility. Final maturation (''
capacitation Capacitation is the penultimate step in the maturation of mammalian spermatozoa and is required to render them competent to fertilize an oocyte. This step is a biochemical event; the sperm move normally and look mature prior to capacitation. ''I ...
'') is completed in the
female reproductive tract The female reproductive system is made up of the internal and external sex organs that function in the reproduction of new offspring. In humans, the female reproductive system is immature at birth and develops to maturity at puberty to be able ...
. The epididymis secretes immobilin, a large glycoprotein that is responsible for the creating of the viscoelastic luminal environment that serves to mechanically immobilize spermatozoa until ejaculation. Immobilin is predominantly secreted into the proximal caput epididymis prior to the acquisition of the potential for sperm motility. During emission, sperm flow from the ''cauda epididymis'' (which functions as a storage reservoir) into the vas deferens where they are propelled by the
peristaltic Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, which ...
action of
muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...
layers in the wall of the vas deferens, and are mixed with the diluting fluids of the prostate, seminal vesicles, and other accessory glands prior to ejaculation (forming semen). Contrary to popular belief, sperm are capable of causing a pregnancy even without ever travelling through the epididymis. This has been proven in two cases in the United States in the 1980s where a couple of men's vasa deferentia were directly surgically attached to their efferent ducts and these men both subsequently impregnated their partners within the next couple of years. This has also been proven in a similar case in Western Europe in the early 1990s.


Antioxidant defenses

During their transit through the epididymis, the spermatozoa undergo a series of transformations in preparation for their ultimate task of fertilizing the oocyte. In order to protect the spermatozoa during their transit through the epididymis, the epididymal
epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
produces a variety of
antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction that can produce free radicals. This can lead to polymerization and other chain reactions. They are frequently added to industrial products, such as fuels and lubricant ...
proteins that help protect the spermatozoa from oxidative damageO'Flaherty C. Orchestrating the antioxidant defenses in the epididymis. Andrology. 2019 Sep;7(5):662-668. doi: 10.1111/andr.12630. Epub 2019 May 1. PMID: 31044545. The antioxidant proteins produced include
catalase Catalase is a common enzyme found in nearly all living organisms exposed to oxygen (such as bacteria, plants, and animals) which catalyzes the decomposition of hydrogen peroxide to water and oxygen. It is a very important enzyme in protecting t ...
, glutathione peroxidases, glutathione-S-transferases, peroxiredoxins,
superoxide dismutase Superoxide dismutase (SOD, ) is an enzyme that alternately catalyzes the dismutation (or partitioning) of the superoxide () radical into ordinary molecular oxygen (O2) and hydrogen peroxide (). Superoxide is produced as a by-product of oxygen me ...
s,
thioredoxin reductase Thioredoxin reductases (TR, TrxR) () are enzymes that reduce thioredoxin (Trx). Two classes of thioredoxin reductase have been identified: one class in bacteria and some eukaryotes and one in animals. In bacteria TrxR also catalyzes the reduction ...
and thioredoxins. Deficiencies in the availability of these antioxidant proteins reduces sperm quality by affecting a variety of the proteins necessary for the motility needed to fertilize oocytes. Reduced antioxidant activity also causes increased oxidative damage to the sperm DNA.


Clinical significance


Inflammation

An inflammation of the epididymis is called
epididymitis Epididymitis is a medical condition characterized by inflammation of the epididymis, a curved structure at the back of the testicle. Onset of pain is typically over a day or two. The pain may improve with raising the testicle. Other symptoms may ...
. It is much more common than testicular inflammation, termed orchitis.


Surgical removal

Epididymotomy is the placing of an incision into the epididymis and is sometimes considered as a treatment option for acute suppurating epididymitis. Epididymectomy is the surgical removal of the epididymis sometimes performed for
post-vasectomy pain syndrome Post-vasectomy pain syndrome (PVPS) is a chronic and sometimes debilitating genital pain condition that may develop immediately or several years after vasectomy. Because this condition is a syndrome, there is no single treatment method, therefore e ...
and for refractory cases of epididymitis. Epididymectomy is also performed for sterilization on some male animals of livestock species so they can be used to detect estrus in females ready for artificially insemination.


Non-human animals

The epididymis is present in all male reptiles, birds, and mammals. In reptiles, there is an additional canal between the testis and the head of the epididymis and which receives the various efferent ducts. This is, however, absent in all birds and mammals.


Gallery

Image:Illu repdt male.jpg, Human male reproductive system Image:Illu testis 1b.jpg, Testis Image:Mesorchium.svg, Schematic drawing: cross-section through a testicle Image:Epididymis high mag.jpg, Micrograph of epididymis - H&E stain Image:Epididymis.JPG, Micrograph File:Slide4UMR.JPG, Deep dissection of epididymis


See also

* Epididymis evolution from reptiles to mammals *


Notes


External links

* {{Authority control Scrotum